1. Field of the Invention
This invention relates to a method of making a spark plug in which a noble metal tip is secured to a front end of a center electrode to impart a spark-erosion resistant property.
2. Description of Prior Art
In a center electrode of a spark plug, a composite structure has been used in which a heat-conductive core (Cu) is embedded in a heat-and erosion-resistant clad metal (nickel-based alloy) as shown in Japanese Patent Publication No. 59-2152. According to the Japanese Patent Publication No. 59-2152, a noble firing tip is further bonded to a front end of the clad metal by means of electric resistance welding so as to improve its spark-erosion resistant property. After completing the electric resistance welding, the front end of the clad metal is milled to make the front end diametrically even with the firing tip.
In the prior art, the electric resistance welding makes it possible to embed the tip in the front end of the clad metal while rounding an edged corner of the firing tip under the influence of the heat and pressure to which the firing tip is subjected.
As a result, a higher voltage is required for the spark plug to establish a spark discharge between its electrodes. Upon cutting the front end of the clad metal in order to reduce the required spark voltage and improve the ignitability, it is unavoidable to mill the firing tip only to fail to make an effective use of the expensive noble metal.
When the front end of the clad metal is eroded to reveal the rounded corner of the firing tip only with a short elapse of service hours, a significantly higher voltage is required for the spark plug to establish the spark discharge between its electrodes.
Therefore, it is one of the objects of the invention to provide a method of making a spark plug which is capable of preventing a buckling collapse of the nobel metal tip, and reducing a required spark voltage while at the same time keeping an edged corner of the tip in good shape.